It is well known that heat destroys microorganisms. The presence of moisture accelerates this destruction by denaturing or coagulation of the proteins making up the microorganisms. Most microorganisms contain sufficient water so that moderate heat alone, e.g. 80° C.-100° C., will destroy the microorganism. Many bacterial spores, on the other hand, contain substantially no water and require elevated temperatures in excess of 150° C. for their destruction where dry heat is used. Hence, the destruction of such organisms is generally carried out in the presence of steam in autoclaves.
Such steam sterilization is generally carried out at temperatures of about 250° F. (121° C.) for at least 12 to 15 minutes or for shorter times at higher temperatures e.g. 270° F. (132° C.). Often, to ensure a sufficient safety margin, times as long as 30 minutes are used. Such long sterilization times give the operator a greater degree of confidence that steam has penetrated throughout the autoclave and among all of its contents. However, such long heat cycles are disadvantageous from the standpoint of economy of time, energy consumption, and severe shortening of the useful life of certain types of sterilized material, e.g., fabric gowns, drapes, and the like.
From time to time attempts have been made to develop sterilization indicators which permit quality control of sterilization with the confidence that all microorganisms have been destroyed. One presently used method is through the use of spore strips or samples. Spores which are particularly difficult to destroy are selected as the control standard, e.g., Bacillus Subtilis var. Niger and Bacillus Stearothermophilus. The spore strip or sample is placed in the autoclave with the materials to be sterilized. At the end of the sterilization cycle, the spore strip or sample is studied to determine whether it is possible to grow organisms in a suitable culture medium. Failure of the spores to reproduce indicates death of spores, and hence, adequate sterilization.
Although this control technique is accurate, it suffers from several inherent disadvantages; (1) excessive cost; (2) delay between processing and control data; (3) batch to batch variation of the spores; and (4) heat resistance of spores decreases with storage time.
Several attempts have been made to devise chemical type sterility indicators. One such product is known as Temp-Tube, and is disclosed in, for example, Kelson, U.S. Pat. No. 3,313,266, White, U.S. Pat. No. 3,341,238, and White, U.S. Pat. No. 3,652,249. The device consists of a sealed tube containing a compound with a melting point which corresponds to the sterilization temperature. The device is capable of indicating whether or not the autoclave was held at a temperature above or below the melting point for a period of time once the melting point is reached.
Other sterility indicators rely on a temperature accelerated chemical reaction to cause color change in an indicator. Though some of these devices are intended to be operative at more than one temperature/time condition, they suffer from the disadvantage that they do not match the spore kill temperature/time relationships. The thermal resistance of spores of a particular species at any temperature is characterized by its temperature coefficient. The symbol Q10 is used to designate the temperature coefficient over a range of 10° C. It corresponds to the ratio of the death rate constant at a particular temperature to the death rate constant at a temperature 10° C. lower. Generally, the measurements are made for a fixed time interval, e.g., 9 minutes.
If the constants at any two temperatures, t1, and a temperature 10° C. higher, t2, are known, then Q10 may be calculated from the equation:log Q10=(10/(t2−t1))×log(K2/K1)
wherein t1 and t2 are as defined and K1 and K2 are the respective death rate constants. Spores generally exhibit a Q10 value of about 10.
Other sterility indicators are known. One such indicator is disclosed in Larsson, U.S. Pat. No. 3,981,683, and uses a backing strip of aluminum foil having an organic compound containing oxygen or nitrogen in contact with a wicking strip, and a cover strip overlying the organic compound and the wicking strip. The cover strip is a polymeric rate controlling film that permits water vapor to pass through at a rate sufficient to make the strip operable at a temperature to be monitored.
One drawback to the device in Larsson is that the temperature and time parameters at which the indictor indicates an acceptable level of sterilization (e.g., that the temperature has been held at a minimum value for a specified period of time) is not well controlled. As such, the indicator can indicate that the requisite level of sterilization has occurred when in fact is has not.
Another such indicator is disclosed in Foley, U.S. Pat. No. 4,448,548. The device in Foley is directed to a steam sterilization indicator in which a fusible material, in tablet form, is deposited in an embossment in an aluminum backing. A wicking strip is attached to the backing in close proximity to the fusible tablet. A clear plastic material covers the tablet and the strip and is adhered to the backing.
The melting point of the fusible tablet is depressed in the presence of saturated steam. Upon melt, the material in the tablet is absorbed by the wicking strip, producing a color front to provide an indication of the integration of time and temperature in the presence of steam. Various amounts of a binder are used in the tablet to provide a device which may be adjusted to reflect the thermal death curves of various types of microorganisms. The cover and the wick are bonded to the backing by an acrylic adhesive which also affects the rate of the indicator.
As with the Larsson device, a drawback to the device in Foley is that the temperature and time parameters at which the indictor indicates an acceptable level of sterilization is not well controlled and as such, the indicator can indicate that the requisite level of sterilization has occurred when in fact is has not.
Therefore, it is desirable to have a sterility indicator that will, in a sense, mimic spore kill. Desirably, such an indicator provides indication that an acceptable level of sterilization has been reached at specific, desired, predetermined time/temperature scenarios.